Atomic layer deposition (ALD) is a layer-by-layer synthesis method capable of depositing conformal thin films with thickness and compositional control on sub-nanometer length scales. ALD is emerging as the premier thin film deposition method to synthesize conformal layers with control over composition and thickness at the angstrom to nanometer length scale. ALD syntheses operate by alternating exposures to complimentary chemical precursor vapors. In the ALD window, each of these precursors reacts with the surface sequentially in a self-limiting fashion to deposit one monolayer or less.
Due to the self-limiting nature of the complimentary surface reactions, conformal pinhole-free films can be deposited on nanostructured surfaces with very high aspect ratios (>1000). See, e.g., Hamann, T. W.; Martinson, A. B. F.; Elam, J. W.; Pellin, M. J.; Hupp, J. T. J. Phys. Chem. C 2008, 112, 10303-10307; see also, Martinson, A. B. F.; Elam, J. W.; Liu, J.; Pellin, M. J.; Marks, T. J.; Hupp, J. T. Nano. Lett. 2008, 8, 2862-2866, both incorporated by reference herein. For photovoltaic solar energy conversion, the unique capabilities of ALD could enable next-generation high efficiency nanostructured devices. One example is plasmon enhancement, which can increase the open-circuit photopotential, see, e.g., Martinson, A. B. F.; Giebink, N. C.; Wiederrecht, G. P.; Rosenmann, D.; Wasielewski, M. R. Energ. Environ. Sci. 2011, 4, incorporated herein by reference, by for example, concentrating photoexcited charge carriers. See, e.g., Warren, S. C.; Thimsen, E. Energ. Environ. Sci. 2012, 5, 5133-5146, incorporated herein by reference. Absorption enhancements based on the localized surface Plasmon resonance (LSPR) are highly localized to within approximately 10 nm of the metal nanoparticle surface, see, e.g., Hagglund, C.; Apell, S. P.; Kasemo, B. Nano. Lett. v2010, 10, 3135-3141; Thimsen, E. Chem. Mater. 2011, 23, 4612-4617; Standridge, S. D.; Schatz, G. C.; Hupp, J. T. J. Am, Chem. Soc. 2009, 131, 8407-8409 incorporated herein by reference, and so conformal absorber layers must be synthesized on the rough substrate at this length scale—a challenging task ideally suited for ALD. Another example where ALD can contribute to improved photovoltaic performance is through minority carrier collection. For semiconductors, in general, the minority carrier collection distance is often incommensurate with the light absorption depth, limiting conversion efficiencies in planar configurations—especially at wavelengths near the band gap for materials with short-lived, low-mobility photoexcited charge carriers.
ALD allows one, in principle, to decouple light absorption from photoexcited charge carrier collection using interpenetrating device geometries. This concept of decoupling light absorption from photoexcited charge carrier (excitons or minority carriers) collection has been proposed in various forms over the years; some classic examples are the dye-sensitized solar cell (DSC), bulk-heterojunction (BHJ) solar cell and extremely thin absorber (ETA) solar cells. The scalable fabrication of a thin absorber layer that exhibits high internal quantum efficiency from earth-abundant, low cost components remains a central challenge to fully realizing the nanostructured device designs.
While many materials have been synthesized by ALD, the technologically-important metal sulfides are underexplored, and homogenous quaternary metal sulfides are absent from the literature.
Compared to metal oxides, metal sulfides have received relatively little attention from the ALD community. Self-limiting chemistry often proceeds readily using H2S as the sulfur source. Beyond binary and ternary metal-sulfides, to the best of our knowledge, there are no examples in the published literature for the synthesis of quaternary sulfides by ALD. Cu2ZnSnS4 (CZTS) is an absorber layer that has attracted attention recently for solar energy conversion because of its band gap (Eg≈1.4 eV), the relative abundance and low cost of its constituent elements, and its demonstrated solar-to-electricity power conversion efficiencies over 8%.11. In CZTS the oxidation states are Cu(+I), Zn(+II), Sn(+IV) and S(−II). Control over composition in this system is important. CZTS compositions in the best devices are Cu-poor and Zn-rich. For example, the 8.4% efficient CZTS device reported by Shin et al. had a Cu/Sn ratio from 1.7 to 1.8, and a Zn/Sn ratio from 1.2 to 1.3.11. It has been proposed that this empirical observation could be a result of the dominant acceptor defect, which is expected to change with composition. In stoichiometric CZTS, the lowest energy acceptor defect has been reported to be the CuZn antisite (i.e. Cu+ sitting on a Zn2+ site), which has a relatively deep acceptor level of 0.12 eV above the valence band maximum. The higher energy Cu+ vacancy defect is more attractive because of its shallower level, only 0.02 eV above the valence band maximum.
The hypothesis is that the Cu-poor, Zn-rich CZTS favors the formation of the Cu vacancy defect and suppresses the CuZn antisite, thereby improving performance. While the hypothesis regarding the role of the dominant acceptor defect in performance remains experimentally untested, the empirical observation of the composition correlation stands. It is clear that a given synthesis process for CZTS must demonstrate control over composition. Even while CZTS devices have exhibited very promising performance, the best devices still have low open-circuit voltages compared to the band gap (Voc≈0.66 V; Eg≈1.4 eV). They also have relatively low quantum efficiencies at wavelengths near the band gap transition (900 nm), as well as below 500 nm where parasitic absorption by the window layers becomes important. The published short circuit photocurrent densities in the highest efficiency CZTS photovoltaic solar cells are approximately 20 mA cm−2, 11 well below the theoretical maximum of 33 mA cm−2 under AM1.5 illumination, leaving room for improvement.
Many synthesis routes to CZTS have been explored. The most common ways to fabricate devices of reasonable efficiency are physical vapor deposition of metal or binary sulfide stacks followed by sulfurization; or solution-phase nanoparticle synthesis followed by deposition onto a substrate and annealing. For physical vapor deposition, a common route is to thermally evaporate metal films (e.g. Cu, Zn and Sn) onto a substrate, and then anneal the metal film stack in a sulfur atmosphere at T>500° C. to generate CZTS. One can also do the same procedure using binary metal sulfide film stacks (e.g. CuS, SnS and ZnS). For nanoparticles, they must be deposited on the substrate by for example, spreading or dip-coating. Like the metal film stacks, nanoparticle deposition is typically followed by annealing in a some type of chalcogen-containing atmosphere to obtain efficient solar cells.